This paper may be read online or downloaded as a PDF at:

  http://digitalcollections.usfca.edu/cdm/ref/collection/p15129coll11/id/148

A PDF version is also available at:

  http://www.trinidadbirding.com/publications/Camras_1953._A_review_of_the_genus_Myopa_in_North_America_Conopidae.pdf
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There are more recent Myopa references for other parts of the world, but I searched through all the published names under Myopa using the "Nomeclator" at Systema Dipteratorum, and found no nearctic taxa there that are missing from this work.

So, based on the above together with other searches, as far as I can tell this is the most recent and only complete revision of *nearctic* species of Myopa. Though Camras & Hurd(1)(1957) was published 4 years later, the treatment of Myopa there (except for the inclusion of range maps) is mostly similar to (or abbreviated based on) the information in this paper. Though focused on California, the latter paper also includes keys and discussions for all current nearctic taxa except M. virginica and two Mexican species, M. fenestrata and M. pulchra.

Though the discussions of taxa in both papers are certainly helpful, some descriptions are not especially detailed and searching out original and/or other descriptions (e.g. written for synonyms) may provide further useful information.